Using EPUB Files, Ideal for any mobile device
EPUB files are ideal for your smartphone or Ipad, just make sure you have an Ereader app installed personally I use a free version FBReader . Once it is installed download or copy the EPUB book files to each device and open the EPUB through the app. Below I have given free books for you to test and see how it works for yourself.
EPUB is my favorite no matter what device you are using because some advantages include the following:
1. Re-flow-able text - no matter your screen size text will adjust to fit that screen so page numbers do not matter
2. Automatic bookmarking - after reading you do not need to bookmark the last page you were reading, next time you open the book it will automatically start off from the same page you had reached (not even a one sentence error)
If you are interested in using FBReader use the following links according to your device platform.
NOTE: After installing the app I recommend you also install its Bookshelf plugin, which displays yours books smartly.
FBReader for iOS go to the App Store. It works on iOS 9.3 and higher.